Shom Hydrography Manuals

Understanding hydrography, hydrographic surveys and marine charting

Shom hydrography manuals provide technical reference material for understanding how the physical characteristics of seas and oceans are measured, controlled and used. They cover subjects such as hydrographic surveys, bathymetry, depth measurement, positioning, data quality and the transformation of survey data into nautical information used for navigation.

Hydrography is not limited to measuring water depth. It also includes the identification of underwater hazards, seabed characteristics, wrecks, obstructions, maritime structures, offshore installations, coastal features and, more broadly, all physical information required to understand the marine environment.

Shom, the French Hydrographic and Oceanographic Service, is France’s reference organization in this field. Its hydrographic data contribute in particular to the production and updating of French nautical documentation. Manuals and publications dedicated to hydrography help explain the methods used to acquire and interpret this information.

What is hydrography?

Hydrography is the science concerned with surveying and describing the physical characteristics of aquatic environments: oceans, seas, coastal areas, lakes and rivers.

In the maritime field, it is directly connected with navigational safety. A nautical chart relies on data that enables navigators to assess depths, hazards, approaches to ports, channels, regulated areas and coastal features.

Shom hydrography manuals therefore help explain the relationship between:

• hydrographic surveying;

• bathymetry;

• depth measurement;

• geographic positioning;

• water levels and reference surfaces;

• hazard detection;

• data processing;

• quality and measurement uncertainty;

• marine charting;

• paper and digital nautical documentation.

This approach is particularly useful for hydrographic professionals, port authorities, engineering and surveying companies, teachers, students and specialists in marine cartography.

Hydrographic surveys: from measurement to data

A hydrographic survey is a data-acquisition campaign carried out at sea or in an aquatic environment to improve knowledge of the physical characteristics of a given area.

Depth is an essential measurement, but it is not the only information collected. A survey can also help identify or position wrecks, obstructions, offshore structures, underwater cables, regulated areas and other features of the seabed and maritime environment.

The techniques used depend on the area being surveyed and the required level of accuracy. Acoustic systems, particularly multibeam echosounders, make it possible to obtain detailed measurements of the underwater relief. Shom also uses other sources of information, including airborne lidar, satellite data and information contained in its hydrographic databases.

Hydrography manuals help explain the different stages of a survey:

• preparation and reconnaissance of the area;

• definition of survey objectives;

• positioning of the vessel or survey platform;

• data acquisition;

• quality control;

• data processing;

• qualification of results;

• integration into hydrographic databases;

• use in marine charting and nautical information.

Bathymetry and depth measurement

Bathymetry is one of the fundamental fields of hydrography. It involves measuring and representing water depths and the relief of the seabed.

Bathymetric knowledge is particularly important in areas where navigation is constrained by depth: approach channels, ports, entrances, anchorages, passages, shoals and coastal waters.

In the Gulf of Morbihan, for example, the combination of bathymetric and topographic data helps characterize a complex coastal environment. Around the approaches to Brest, Lorient, Saint-Nazaire and La Rochelle, knowledge of the seabed contributes directly to understanding access and navigation conditions.

Bathymetry also applies to deeper ocean environments, including the Atlantic Ocean, Mediterranean Sea, Indian Ocean and Pacific Ocean.

Data may be acquired at different scales and using different technologies. Their accuracy depends on the survey method, positioning system, environmental conditions, water depth and equipment used.

Tides, reference levels and positioning

A measured depth has meaning only when it is associated with a properly defined vertical reference system.

Hydrography must therefore take into account tides, variations in sea level, vertical reference surfaces and the geodetic systems used to position survey data.

Horizontal positioning is also an essential component. GNSS systems can provide highly accurate positioning, but their use must be associated with a rigorous understanding of reference systems and measurement uncertainty.

Hydrography manuals therefore help explain why a depth shown on a nautical chart does not necessarily correspond to the instantaneous water depth observed in the field.

This distinction is essential when correctly interpreting soundings, depth contours and bathymetric information.

Data quality, accuracy and uncertainty

One of the main objectives of hydrography is to produce data whose quality is known and controlled.

A hydrographic survey is therefore not simply a matter of recording depths. It also requires the accuracy of measurements to be assessed, sources of error to be identified and the consistency of the resulting data to be verified.

International hydrographic standards, including standards developed by the International Hydrographic Organization (IHO), establish requirements appropriate to different survey purposes.

Shom manuals and technical documentation help explain concepts such as:

• horizontal and vertical accuracy;

• measurement uncertainty;

• resolution;

• sounding density;

• quality control;

• anomaly detection;

• data processing;

• survey qualification;

• metadata;

• reference surfaces.

This knowledge is particularly important when data from different sources need to be compared or combined.

From hydrography to nautical charts

One of the main applications of hydrographic data is marine charting.

Survey information is analysed, controlled and integrated into databases used to produce or update nautical documentation.

A nautical chart may represent:

• depths and soundings;

• navigational hazards;

• the coastline;

• landmarks;

• aids to navigation;

• wrecks;

• maritime structures;

• regulated areas;

• offshore installations;

• harbour approaches.

This production chain explains why the quality of a nautical chart depends directly on the quality of the hydrographic data on which it is based.

Knowledge acquired through hydrography manuals therefore provides a better understanding of charts produced by Shom, as well as hydrographic documentation issued by other national hydrographic offices.

Hydrography along the French coast and in major maritime areas

Hydrographic activity covers highly diverse environments.

Along the French Atlantic and English Channel coasts, areas around Brest, Cherbourg, Le Havre, Dieppe, Saint-Malo, La Rochelle, Bordeaux and Bayonne have very different characteristics in terms of depth, tides, currents and maritime traffic.

In the Mediterranean, the approaches to Marseille, Toulon, Nice, Ajaccio and Bastia present different bathymetric and oceanographic conditions.

Internationally, hydrographic methods are also applied in the North Sea, Baltic Sea, Atlantic Ocean, Mediterranean Sea, Indian Ocean, Pacific Ocean and polar regions.

Hydrographic data produced by different national services can subsequently contribute to international marine charting activities.

Shom, Admiralty, Imray and NV Charts: understanding the different players

Shom is the French reference hydrographic service. Its data and publications cover French waters and associated maritime territories under French responsibility.

Internationally, other organizations and publishers play an important role in nautical documentation. The United Kingdom Hydrographic Office (Admiralty) is, for example, a major reference for British waters and numerous international maritime areas.

NV Charts also produces charting documentation for navigation, particularly in Europe, while Imray has historically been associated with numerous coastal and offshore sailing areas, especially in Europe and certain tropical regions.

These organizations do not necessarily produce the same types of data or follow identical editorial processes. Understanding hydrography therefore helps users better appreciate the differences between hydrographic offices, databases, nautical charts and maritime publications.

Who are Shom hydrography manuals intended for?

Hydrography manuals may be useful to a wide range of readers:

• hydrographers and specialist technicians;

• maritime engineering and surveying companies;

• port authorities;

• engineers and geospatial specialists;

• hydrography students and teachers;

• marine cartography professionals;

• research organizations;

• maritime data managers;

• bathymetry specialists;

• maritime professionals;

• navigators wishing to deepen their understanding of nautical documentation.

They are also useful for understanding why cartographic information has a defined accuracy, scale, date and level of reliability.

Why consult a Shom hydrography manual?

A hydrography manual makes it possible to move beyond simply reading a nautical chart and develop a deeper understanding of the production chain behind nautical information.

It helps explain:

• how a depth is measured;

• how a hydrographic survey is organized;

• how survey data are positioned;

• how tides are taken into account;

• how hazards are detected;

• how survey quality is assessed;

• how data are transformed into cartographic information;

• why some information has inherent limitations;

• how to interpret bathymetric data correctly.

The Shom publication devoted to hydrography, nautical documents, their imperfections and their proper use specifically addresses hydrographic surveys, their accuracy, imperfections in nautical documentation, and the use of GPS and electronic charts.

Conclusion

Shom hydrography manuals provide an understanding of the scientific and technical foundations of modern marine charting. They explain the transition from observation of the marine environment to hydrographic surveying, bathymetric measurement, data processing and the production of nautical documentation.

For professionals, students and experienced users of maritime documentation, this knowledge provides a better understanding of the accuracy, limitations and construction of the information represented on nautical charts.

Hydrography is therefore an essential link between knowledge of the physical characteristics of seas and oceans and navigational safety, from the port approaches of Brest, Cherbourg, Marseille and La Rochelle to the vast maritime areas of the Atlantic Ocean, Mediterranean Sea, Indian Ocean and Pacific Ocean.

FAQ – Shom Hydrography Manuals

What is a Shom hydrography manual?

It is a technical publication covering the methods, data and principles used to measure and describe the underwater environment and produce the hydrographic information required for navigation.

What is hydrography used for?

Hydrography is used to determine depths, underwater relief, hazards, structures, wrecks and other physical characteristics required for safe navigation.

What is the difference between hydrography and bathymetry?

Bathymetry primarily concerns the measurement and representation of water depths and seabed relief. Hydrography has a broader scope and also includes other physical information required to understand and chart marine environments.

How are hydrographic surveys carried out?

They can use acoustic sounders, multibeam systems, GNSS positioning, airborne lidar and various complementary sources of data.

Why are tides important in hydrography?

Because sea level changes over time. Depth measurements must therefore be related to an appropriate reference level so that they can be correctly interpreted and used for marine charting.

Are hydrographic data used only to produce nautical charts?

No. They can also be used for port infrastructure, marine spatial management, scientific research, environmental monitoring, modelling and many other maritime applications.

Which hydrographic services produce information comparable to Shom?

Many countries have their own national hydrographic service. The United Kingdom, for example, has the United Kingdom Hydrographic Office and its Admiralty publications, while other national organizations produce data for their respective maritime areas.

Are hydrography manuals intended only for professional hydrographers?

No. Although they contain technical material, they can also be useful to students, teachers, engineers, port professionals, cartographers, geospatial specialists and navigators who want to understand the origin and limitations of the information presented in nautical documentation.